4wd, tire blowouts, and different size tires...did I just f*** up?

Hey all

Owner of 707/2000 since August. Everything was fine with my car until yesterday. Was at school, come out and find that there was a puncture in the sidewall of my rear driver tire. Having no spare, I called around different friends till I found someone who shared the same bolt pattern as me. Unfortunately, the spare was only a 14 in tire, knowing that people commonly say that you cant run different sized rims/tires on 4wd vehicles, I was hesitant to even do so.

I made it about 5 of the 25 mile trip back to my house, when the spare went flat. Luckily just a few hundred feet from a friend's house. I gave up for the night at that point, as 9 pm miraculously became 12 am.

Enter today. I find a friend who has a full set of rims with 195/50/15s that he can give me. Lucky break, or so I thought at the time, as these rims led to a completely unrelated set of problems. Come four hours later, and it's now 10 o'clock pm. I'm finally getting the third of the four tires on, and drop the car to tighten the bolts. I hear a hiss come from out of the newly mounted tire, and nearly began to cry. Luckily (or unluckily), the tire popped off the bead, and was just leaking air from not beng sealed properly. To make matters worse, only three of the four directional tires I was now in possession of were mounted facing the proper direction (who does that?).

At this point, I am left with no remaining options. I had to drive the car home. I had two 195/60/15's in the front, and two 195/50/15's in the rear. The latest 40 miles of my car's life has been spent driving on mismatched tires... Did I do any significant damage to my awd system?

 

[fuel]

definitely not good for the drivetrain, the viscous coupling in the center diff was probably getting a work out.

 

[Terry Posten]

At the very least, use all the same size tires. They can be any size that you have as long as they fit in the wheel well and they are the same all the way around.

The best thing to have is a matched set of 4. Same make, model, and size.

Do not drive any further until you get the same sized tires on all 4 corners. The risk to the tranny and t-case can cost you thousands to fix.

 

[gramkrakr89]

Differentials in general don't like uneven wheel diameters across them... In this case, the viscous coupling inside the tranny was probably getting quite toasty on your drive home, hopefully everything is still alright.

 

Are there any certain signs of trouble? I got the full set of matching wheels and tires on today and drove the car for about 15 minutes, everything seemed to be in proper running order, but I dunno how my car would feel if it was riding broken anyway

 

[LIV4PSI]

Diffs usually make plenty of noise if they break, I'm sure you would know. It may have caused some premature wear, but I would think if it was going to break it would have.

Glad you haven't had any other problems. Good luck with the new car!

 

[fuel]

it will probably be OK - Mitsubishis seem to be a bit more tolerant than say a Subaru where even uneven tread wear between front and rear can cause hopping/skipping and binding up. The viscous coupling in the center diff has probably been worn a bit more than it usually would have but it certainly wouldn't have physically broken anything.

I picked up a RVR (Expo LRV) AWD which had 196/65 on the front and 205/65 on the rear and had been like that with the previous owner for some time and it seemed to still drive fine with the mis-matched tyre size and when I put correct sized tyres all round.

 

[omertaitalia]

^ 196 and 205 have to do with width, if the rear is wider then the front it doesn't matter.

 

[jepherz]

^ Because the high of the sidewall is 65% of the width, so those tires were different rolling diameters.

 

[Terry Posten]

Exactly as Jeff stated, the tire sizing guide is all based on aspect ratios.

The first number is the width in mm. The second number is the side wall height as a percentage of the tread width.

So, a 195/65/15 has a sidewall height of 126.75mm. A 205/65/15 has a sidewall height of 133.25mm.

 

[jepherz]

Obviously the AWD system on these cars can cope with some amount of difference between rolling diameters or they wouldn't bother putting spare tires in the trunk. However, distance and speed play a huge role in whether or not you do actual damage. The two parts within the transmission that directly suffer are the center diff and the viscous coupler unit. If these get destroyed, the fix is less than $150, however, that isn't to say you couldn't have still done more damage.

I guess I would change the oil and start from there. If it's fairly clean and the car still turns tight corners without acting as if it has a welded diff, you're probably fine.

 

[RayH]

Tire aspect will get you in the ballpark and then revolutions per mile that you can find at TireRack will get you the closest match.

 

[Whoodoo]

Chucking a spare on shouldn't do any damage at all, especially if you have the stock open diffs front and rear. Its just like going around a corner; the smaller diameter wheel will be spinning its drive axle faster, but the driveshaft will not be rotating any faster.

The wear on the center diff only happens when you force the rear axle to rotate faster or slower than the front axle due to having different tires on both wheels.

 

[jepherz]

I disagree. The combined rear diameter should match the combined fronts or your center diff Is doing something.